The invention relates to methods and apparatus for forming threads on a workpiece and, in particular, to such methods and apparatus for cutting tapered threads on bar, pipe, or like stock.
In general, prior art power-operated retreating die thread cutting equipment has been relatively complex, expensive to build, slow in operation, and inefficient in power consumption. In many instances, prior devices have not been suitable for use in cutting tapered threads on reinforcing bar, since such devices require rotation and/or axial feeding of the workpiece. This requisite movement of a workpiece can be impractical in applications such as reinforcing bar threading because of the relatively long lengths and mass of the workpieces being processed.
U.S. Pat. No. 2,054,028 to Benninghoff discloses a power-operated machine for cutting tapered threads on pipe. The machine incorporates a retreating die mechanism which is relatively complex. U.S. Pat. No. 3,812,548 to Theuerkaue shows a machine for cutting tapered threads on a pipe which has a threading head driven in rotation in timed relation to the axial feed rate of a workpiece. U.S. Pat. No. 3,864,774 to Fohl discloses apparatus for cutting tapered pipe threads which employs a scroll plate or control disc to control die retreating movement U.S. Pat. No. 1,116,838 to Oster et al. discloses thread cutting apparatus in which die retreating action is effected by interengaged, axially arranged screw elements. U.S. Pat. No. 4,526,496 to Kies et al. discloses a threading head for cutting tapered threads on a reinforcing bar. U.S. Pat. No. 4,282,618 to Wagner discloses a die for cutting tapered threads on reinforcing bars for concrete, the die being characterized by a relatively long total cutting edge length.